[v6r10][NEW ]New request system everywhere

Solving https://github.com/DIRACGrid/DIRAC/issues/1523

NB:
- The changes are quite simple, I made small unit tests, but I didn't make any integration test, that should be done in certification, in particular:
  - Test the replication, which means testing the chain TS -> RMS -> FTS
  - Test that accounting requests are well formed and correctly treated.
  - Test that requests from the JobCleaningAgent are well formed and correctly treated.
- I marked LFCvsSEAgent and LFCvsSEAgent as abandonware: I don't think any extension is using them, please tell me if I am wrong
- https://github.com/DIRACGrid/DIRAC/issues/1621 will come in the future.